For many years pesticides, particularly herbicides, were sold as liquids, for example, in 5 gallon steel cans. These cans are very expensive and an environmental nuisance. As a result, it has become the practice to produce pesticides in a dry, flowable form so that the pesticides can be packaged in less expensive, disposable bags.
Three conventional methods for producing dry water-soluble or water-dispersible pesticidal material are in current use. All are at least two-step processes: water is added, the granules are formed and then the free water is driven off. In the primary prior art method, pan granulation, from about 5 to 10% free water is added to the pesticide before granulation, yielding a total water content, before drying, of from about 18 to 22%. Unfortunately, this process produces round particles which roll readily and disperse freely in the environment if bags are broken or accidentally spilled. Also, the particles of this process have a broad particle size spectrum, i.e., many fines and oversized particles, with overall bulk densities of from only about 22 to 28 pounds per cubic foot.
The second method used involves spray drying and the disadvantages of large volume equipment and low production rates. Furthermore, high energy input is needed to eliminate the large amount of water added during processing (up to 80%). Particles produced by this process are also spherical.
The third means of producing dry flowable material utilizes an extrusion wherein the pesticidal composition is mixed in an extruder, extruded through a die, and cut into pellets. No chemical reaction occurs in the extruder The products are usually composed of very hard, compact pellets which are undesirable for water-soluble formulations for the very hardness and compactness of the pellets make them difficult to dissolve.
Such procedures are described in U.S. Pat. Nos. 2,992,090; 3,062,637; 4,065,289; 4,374,082; and 4,435,383 and published U.K. Patent Application Nos. 2,094,624 and 2,109,687.